mo mo ko

Enables all three of these - weapon selection, stage selection, and limited invulnerability - as well as cutscene skipping (press A to skip it)

mu te ki

Limited invulnerability (some things like pits can still kill you)

he i ki

Weapon select (press Select during gameplay to change weapons, or twice in a row in the overworld)

sa ka ta

Weapon select AND stage select (pause the game and press Select for the overworld stages and A for other in-game events)